1. Field of the Invention
The present invention relates to an adhesive and more particularly to a moisture curable adhesive. The present invention is further directed to a method of joining two adherends.
2. Technology Review
Adhesives are commonly used to join or fasten two or more adherends. Adherends are considered as being any two or more materials, or pieces of material that are being joined together, including wood, metals, paper, ceramics, stone, glass, concrete, etc. Adhesives used for these purposes are based on a wide range of technologies, including elastomer/solvent/resin mixtures, epoxies, latexes, polyurethanes, silicones, cyanoacrylates, acrylics, hot melts, and others. These adhesives each have a number of drawbacks.
Adhesives based on elastomer/solvent/resin mixtures, henceforth referred to as rubber-based adhesives, contain solvents which are toxic and, most often, flammable. Despite these hazards, rubber-based adhesives are popular due to their ease of use. Epoxy, acrylic, some polyurethane, and some silicone adhesives are formulated into two-component systems consisting of a base containing monomers, oligomers, and polymers, and a hardener or curative component, consisting of catalysts and curing agents. These systems require accurate measuring and thorough mixing to develop the properties that make them useful as adhesives. The individual raw materials in two-component adhesive products, monomers, oligomers, catalysts, and curing agents, can be toxic and often allergenic. Latex adhesives are low toxicity, non-flammable materials, which harden through evaporation of water. While effective in many applications, latex adhesives do not perform well in low temperature environments, on metals, on many plastics, and in wet or humid environments. In conditions of low temperature or high humidity, the drying time of latex adhesives can be greatly extended, sometimes for days. On the other hand, hot melt adhesives, being thermoplastic by definition, lose strength in warm conditions, limiting the applications in which they can be used. Additionally, hot melt adhesives require a source of heat for application, which further limits their use. These heat sources and the application of hot adhesive present a safety (burn) hazard to the user. Single component adhesives, such as polyurethanes and silicones, cure by reaction with moisture in air or on the adherends. Polyurethanes develop tack slowly, requiring that the adherends be held or fixtured in place (bracing or taping) until the adhesive has cured enough to hold them in place without being held. The same applies for single component silicone adhesives. Further, in a portion of the population, allergic reactions can result from exposure to polyurethane adhesives. In addition to the problems listed above, solvent and water based adhesives generally exhibit a high degree of shrinkage due to the volatile content of the adhesives.
The longevity and durability of adhesive bonding is dependent on a number of factors, including but not limited to surface properties of the adherends, the type and properties of the adhesive used in the bonding application, environmental conditions (temperature, humidity, etc.), the design of the adhesive joint, and the method of joining or assembling the adhesive and adherends together. High solids content adhesives solve some of the problems encountered with these other types of adhesives (i.e., shrinkage, amount of volatiles, etc), however the best of these adhesives present other drawbacks. For instance, European Patent 0 442 380 B 1 describes the use of a high solids content moisture curable adhesive with which two adherends can be bonded together. The patent description, however, indicates if the adhesive “is applied to thickly, the adhesive cannot form a sufficient tacking force (tackiness), and becomes weak in the contact ability. It is therefore preferable that the adhesive be applied in the thickness of approximately 1 mm or less so as to increase the tack.”
The need to apply a thin layer of adhesive limits the applications an adhesive can be used for, and the means (i.e., roller, etc) for applying the adhesive. To apply thin layers of adhesives it is helpful to have an adhesive that can maintain its adhesive properties, and will spread and flow across the substrate or adherend. If the viscosity of the adhesive is too high it can be difficult or impossible to apply a thin layer of adhesive (layers of 1 mm or less). One of the factors which influences the viscosity of an adhesive is the amount of filler in the adhesive. In general, the higher the filler content the higher the viscosity. Filler content also affects the tack of the adhesive. At higher filler contents, the tack is lower. This affect is even more pronounced when applying thin layers of adhesive resulting in an even greater reduction in the tack of the adhesive. Therefore, to get a high solids content adhesive with sufficient tack and a low enough viscosity to spread thinly it has been necessary to use formulations with relatively low filler content. Further, it has been difficult to even add filler to high solids content clear adhesives since most fillers including clear fillers will either block or refract light passing through an adhesive making the adhesive appear unclear. There is, however, a great need to increase the filler content of an adhesive since a higher filler content lowers the cost of an adhesive and reduces the environmental impact of manufacturing the adhesive.
Another problem associated with most good adhesives has been the repositionability of the adherends bonded by the adhesives. Conventionally, once an adhesive had developed enough tack to bond two adherends without clamps or some other type of temporary fasteners, then it was difficult to reposition the adhesive after contacting the adherends after more than a couple minutes. A need therefore exists for an adhesive, which develops tack rapidly so the two adherends to be bonded by the adhesive can be joined and held together without the use of clamps after a relatively short period of time, but where after joining the two adherends can be repositioned with respect to one another for a relatively long period of time.
It is therefore two of the objects of the present invention among the many objects of this invention to solve these problems.